Re: [PATCH v14 16/18] vmpressure: in-kernel notifications

Vladimir Davydov
2013-12-20T14:36:29Z

On 12/20/2013 06:31 PM, Glauber Costa wrote:
>> I have the exact problem described above for a project I'm working on
>> and this solution seems to solve it well.
>>
>> However, I had a few issues while trying to use this interface. I'll
>> comment on them below, but please take this more as advice seeking
>> than patch review.
>>
>>> This patch extends that to also support in-kernel users. Events that
>>> should be generated for in-kernel consumption will be marked as such,
>>> and for those, we will call a registered function instead of triggering
>>> an eventfd notification.
>>>
>>> Please note that due to my lack of understanding of each shrinker user,
>>> I will stay away from converting the actual users, you are all welcome
>>> to do so.
>>>
>>> Signed-off-by: Glauber Costa <glommer@openvz.org>
>>> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
>>> Acked-by: Anton Vorontsov <anton@enomsg.org>
>>> Acked-by: Pekka Enberg <penberg@kernel.org>
>>> Reviewed-by: Greg Thelen <gthelen@google.com>
>>> Cc: Dave Chinner <dchinner@redhat.com>
>>> Cc: John Stultz <john.stultz@linaro.org>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>> Cc: Michal Hocko <mhocko@suse.cz>
>>> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>> ---
>>> include/linux/vmpressure.h | 5 +++++
>>> mm/vmpressure.c | 53 +++++++++++++++++++++++++++++++++++++++++---
>>> 2 files changed, 55 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
>>> index 3f3788d..9102e53 100644
>>> --- a/include/linux/vmpressure.h
>>> +++ b/include/linux/vmpressure.h
>>> @@ -19,6 +19,9 @@ struct vmpressure {
>>> /* Have to grab the lock on events traversal or modifications. */
>>> struct mutex events_lock;
>>>
>>> + /* False if only kernel users want to be notified, true otherwise. */
>>> + bool notify_userspace;
>>> +
>>> struct work_struct work;
>>> };
>>>
>>> @@ -38,6 +41,8 @@ extern int vmpressure_register_event(struct cgroup_subsys_state *css,
>>> struct cftype *cft,
>>> struct eventfd_ctx *eventfd,
>>> const char *args);
>>> +extern int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
>>> + void (*fn)(void));
>>> extern void vmpressure_unregister_event(struct cgroup_subsys_state *css,
>>> struct cftype *cft,
>>> struct eventfd_ctx *eventfd);
>>> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
>>> index e0f6283..730e7c1 100644
>>> --- a/mm/vmpressure.c
>>> +++ b/mm/vmpressure.c
>>> @@ -130,8 +130,12 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
>>> }
>>>
>>> struct vmpressure_event {
>>> - struct eventfd_ctx *efd;
>>> + union {
>>> + struct eventfd_ctx *efd;
>>> + void (*fn)(void);
>> How does the callback access its private data?
>>
>>> + };
>>> enum vmpressure_levels level;
>>> + bool kernel_event;
>>> struct list_head node;
>>> };
>>>
>>> @@ -147,12 +151,15 @@ static bool vmpressure_event(struct vmpressure *vmpr,
>>> mutex_lock(&vmpr->events_lock);
>>>
>>> list_for_each_entry(ev, &vmpr->events, node) {
>>> - if (level >= ev->level) {
>>> + if (ev->kernel_event) {
>>> + ev->fn();
>> I think it would be interesting to pass 'level' to the callback (I'll
>> probably use it myself), but we could wait for a in-tree user before
>> adding it.
>>
>>> + } else if (vmpr->notify_userspace && level >= ev->level) {
>>> eventfd_signal(ev->efd, 1);
>>> signalled = true;
>>> }
>>> }
>>>
>>> + vmpr->notify_userspace = false;
>>> mutex_unlock(&vmpr->events_lock);
>>>
>>> return signalled;
>>> @@ -222,7 +229,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
>>> * we account it too.
>>> */
>>> if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
>>> - return;
>>> + goto schedule;
>>>
>>> /*
>>> * If we got here with no pages scanned, then that is an indicator
>>> @@ -239,8 +246,15 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
>>> vmpr->scanned += scanned;
>>> vmpr->reclaimed += reclaimed;
>>> scanned = vmpr->scanned;
>>> + /*
>>> + * If we didn't reach this point, only kernel events will be triggered.
>>> + * It is the job of the worker thread to clean this up once the
>>> + * notifications are all delivered.
>>> + */
>>> + vmpr->notify_userspace = true;
>>> spin_unlock(&vmpr->sr_lock);
>>>
>>> +schedule:
>>> if (scanned < vmpressure_win)
>>> return;
>>> schedule_work(&vmpr->work);
>>> @@ -324,6 +338,39 @@ int vmpressure_register_event(struct cgroup_subsys_state *css,
>>> }
>>>
>>> /**
>>> + * vmpressure_register_kernel_event() - Register kernel-side notification
>>> + * @css: css that is interested in vmpressure notifications
>>> + * @fn: function to be called when pressure happens
>>> + *
>>> + * This function register in-kernel users interested in receiving notifications
>>> + * about pressure conditions. Pressure notifications will be triggered at the
>>> + * same time as userspace notifications (with no particular ordering relative
>>> + * to it).
>>> + *
>>> + * Pressure notifications are a alternative method to shrinkers and will serve
>>> + * well users that are interested in a one-shot notification, with a
>>> + * well-defined cgroup aware interface.
>>> + */
>>> +int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
>>> + void (*fn)(void))
>>> +{
>>> + struct vmpressure *vmpr = css_to_vmpressure(css);
>> This doesn't allow for css=NULL. What's the recommended way for a today's
>> shrinker (which is not related to cgroups) to register with this API?
>>
>> Also, you don't seem to provide a way to de-register from the event.
>>
> The answer for all of your questions above can be summarized by noting
> that for the lack of other users (at the time), this patch does the bare minimum
> for memcg needs. I agree, for instance, that it would be good to pass the level
> but since memcg won't do anything with thta, I didn't pass it.
>
> That should be extended if you need to.
>
>> I hacked a patch to be able to use this, seems to work but it's a ugly
>> hack:
>>
>> ---
>> include/linux/vmpressure.h | 3 ++-
>> mm/vmpressure.c | 13 +++++++++----
>> 2 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
>> index 9102e53..de416b6 100644
>> --- a/include/linux/vmpressure.h
>> +++ b/include/linux/vmpressure.h
>> @@ -42,7 +42,8 @@ extern int vmpressure_register_event(struct cgroup_subsys_state *css,
>> struct eventfd_ctx *eventfd,
>> const char *args);
>> extern int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
>> - void (*fn)(void));
>> + void (*fn)(void *data, int level),
>> + void *data);
>> extern void vmpressure_unregister_event(struct cgroup_subsys_state *css,
>> struct cftype *cft,
>> struct eventfd_ctx *eventfd);
>> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
>> index 730e7c1..4ed0e85 100644
>> --- a/mm/vmpressure.c
>> +++ b/mm/vmpressure.c
>> @@ -132,9 +132,10 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
>> struct vmpressure_event {
>> union {
>> struct eventfd_ctx *efd;
>> - void (*fn)(void);
>> + void (*fn)(void *data, int level);
>> };
>> enum vmpressure_levels level;
>> + void *data;
>> bool kernel_event;
>> struct list_head node;
>> };
>> @@ -152,7 +153,7 @@ static bool vmpressure_event(struct vmpressure *vmpr,
>>
>> list_for_each_entry(ev, &vmpr->events, node) {
>> if (ev->kernel_event) {
>> - ev->fn();
>> + ev->fn(ev->data, level);
>> } else if (vmpr->notify_userspace && level >= ev->level) {
>> eventfd_signal(ev->efd, 1);
>> signalled = true;
>> @@ -352,21 +353,25 @@ int vmpressure_register_event(struct cgroup_subsys_state *css,
>> * well-defined cgroup aware interface.
>> */
>> int vmpressure_register_kernel_event(struct cgroup_subsys_state *css,
>> - void (*fn)(void))
>> + void (*fn)(void *data, int level), void *data)
>> {
>> - struct vmpressure *vmpr = css_to_vmpressure(css);
>> + struct vmpressure *vmpr;
>> struct vmpressure_event *ev;
>>
>> + vmpr = css ? css_to_vmpressure(css) : memcg_to_vmpressure(NULL);
>> +
>> ev = kzalloc(sizeof(*ev), GFP_KERNEL);
>> if (!ev)
>> return -ENOMEM;
>>
>> ev->kernel_event = true;
>> + ev->data = data;
>> ev->fn = fn;
>>
>> mutex_lock(&vmpr->events_lock);
>> list_add(&ev->node, &vmpr->events);
>> mutex_unlock(&vmpr->events_lock);
>> +
> Your patch makes sense.

I guess I'll include this to the next iteration of this patch-set then.

Thanks.